editor: Add minimap (#26893)

## Overview

This PR adds the minimap feature to the Zed editor, closely following
the [design from Visual Studio
Code](https://code.visualstudio.com/docs/getstarted/userinterface#_minimap).
When configured, a second instance of the editor will appear to the left
of the scrollbar. This instance is not interactive and it has a slimmed
down set of annotations, but it is otherwise just a zoomed-out version
of the main editor instance. A thumb shows the line boundaries of the
main viewport, as well as the progress through the document. Clicking on
a section of code in the minimap will jump the editor to that code.
Dragging the thumb will act like the scrollbar, moving sequentially
through the document.

![screenshot of Zed with three editors open and the minimap enabled,
showing the
slider](https://github.com/user-attachments/assets/4178d23a-a5ea-4e38-b871-06dd2a8f9560)

## New settings

This adds a `minimap` section to the editor settings with the following
keys:

### `show`

When to show the minimap in the editor.
This setting can take three values:
1. Show the minimap if the editor's scrollbar is visible: `"auto"`
2. Always show the minimap: `"always"`
3. Never show the minimap: `"never"` (default)

### `thumb`

When to show the minimap thumb.
This setting can take two values:
1. Show the minimap thumb if the mouse is over the minimap: `"hover"`
2. Always show the minimap thumb: `"always"` (default)

### `width`

The width of the minimap in pixels.

Default: `100`

### `font_size`

The font size of the minimap in pixels.

Default: `2`

## Providing feedback

In order to keep the PR focused on development updates, please use the
discussion thread for feature suggestions and usability feedback: #26894


## Features left to add

- [x] fix scrolling performance
- [x] user settings for enable/disable, width, text size, etc.
- [x] show overview of visible lines in minimap
- [x] clicking on minimap should navigate to the corresponding section
of code
- ~[ ] more prominent highlighting in the minimap editor~
- ~[ ] override scrollbar auto setting to always when minimap is set to
always show~

Release Notes:

- Added minimap for high-level overview and quick navigation of editor
contents.

---------

Co-authored-by: MrSubidubi <dev@bahn.sh>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/scroll.rs

@@ -157,6 +157,8 @@ pub struct ScrollManager {
     active_scrollbar: Option<ActiveScrollbarState>,
     visible_line_count: Option<f32>,
     forbid_vertical_scroll: bool,
+    dragging_minimap: bool,
+    show_minimap_thumb: bool,
 }
 
 impl ScrollManager {
@@ -172,6 +174,8 @@ impl ScrollManager {
             last_autoscroll: None,
             visible_line_count: None,
             forbid_vertical_scroll: false,
+            dragging_minimap: false,
+            show_minimap_thumb: false,
         }
     }
 
@@ -341,6 +345,24 @@ impl ScrollManager {
         self.show_scrollbars
     }
 
+    pub fn show_minimap_thumb(&mut self, cx: &mut Context<Editor>) {
+        if !self.show_minimap_thumb {
+            self.show_minimap_thumb = true;
+            cx.notify();
+        }
+    }
+
+    pub fn hide_minimap_thumb(&mut self, cx: &mut Context<Editor>) {
+        if self.show_minimap_thumb {
+            self.show_minimap_thumb = false;
+            cx.notify();
+        }
+    }
+
+    pub fn minimap_thumb_visible(&mut self) -> bool {
+        self.show_minimap_thumb
+    }
+
     pub fn autoscroll_request(&self) -> Option<Autoscroll> {
         self.autoscroll_request.map(|(autoscroll, _)| autoscroll)
     }
@@ -396,6 +418,15 @@ impl ScrollManager {
         }
     }
 
+    pub fn is_dragging_minimap(&self) -> bool {
+        self.dragging_minimap
+    }
+
+    pub fn set_is_dragging_minimap(&mut self, dragging: bool, cx: &mut Context<Editor>) {
+        self.dragging_minimap = dragging;
+        cx.notify();
+    }
+
     pub fn clamp_scroll_left(&mut self, max: f32) -> bool {
         if max < self.anchor.offset.x {
             self.anchor.offset.x = max;